Holy crap that crank got fucked! I'd definitely check the oil pump and make sure you're getting 2 stroke oil to the cylinder. Looks like the big end bearings seized up on the conrod?
IMO the stock cylinder is best. If you get the 70cc you will want to adjust the cylinder head to match the squish band, and you will have to retune the carb (which you might need to do anyway if that crank and piston damage was caused by a lean condition.)